Which construct is a Message Authentication Code used to verify data integrity and authenticity with a secret key?

• A. MAC (Message Authentication Code)
• B. Digital Signature
• C. Nonce
• D. Initialization Vector (IV)

Answer: (A)

A Message Authentication Code uses a secret key to verify data integrity and authenticity by producing a short tag from the message and the shared key. The recipient, who also knows the secret key, recomputes the MAC on the received data and checks the tag; if they match, the data hasn’t been altered and likely came from someone who possesses the key. This relies on a shared secret, so verification is limited to trusted parties who know the key. This is different from a digital signature, which uses public/private keys and allows anyone with the public key to verify the signature, providing non-repudiation. Nonces prevent replay, and initialization vectors randomize encryption; neither serves as a secret-key-based integrity/authenticity tag.